The sharpen tool is the seventh tool down from the top, on the left. It's in with blur and smudge. Hold down that button to pop the hidden tools out. You can also cycle through them using SHIFT+R (on Mac. probably the same on Win but I'm not sure) Sharpen looks like a triangle. I'm assuming this is the sharpen tool you are talking about as it's the only one that can be applied with a brush. It only sharpens however, giving the pixels a harder less aliased edge. It does not affect contrast.